Luteinizing hormone (LH) is a reproductive hormone that plays a central role in the menstrual cycle. This hormone is produced by the pituitary gland and regulates the function of the ovaries, making its timing a useful tool for fertility tracking. A distinct, rapid elevation in its level, known as the LH surge, pinpoints the time when an egg is about to be released. Pinpointing this event allows for the accurate prediction of the most fertile days in a woman’s cycle.
The Function of Luteinizing Hormone in Fertility
The rise in LH is the direct hormonal trigger for the final stages of egg maturation and release. During the follicular phase, LH stimulates ovarian follicles to produce estradiol, a form of estrogen. As the dominant follicle matures, it secretes high amounts of estrogen, which circulates back to the brain. When estradiol levels remain elevated, they trigger a positive feedback signal to the pituitary gland. This causes the gland to release a large, acute amount of LH, which is the direct signal that causes the mature follicle to rupture, releasing the egg in a process called ovulation.
Defining the LH Surge and Peak
The terms “LH surge” and “LH peak” describe different, though closely related, points in this hormonal event. The LH surge refers to the initial, rapid rise in luteinizing hormone concentration from its baseline level. This surge marks the beginning of the most fertile period of the cycle.
The LH peak is the moment the hormone reaches its highest concentration during that surge window. While the surge indicates that ovulation is imminent, the peak provides the most precise point for predicting the egg’s release. The timing of ovulation is tied to the maximum concentration of the hormone. Although the surge can be gradual or rapid, the single LH peak remains the most common and definitive pattern observed.
Timing: How Long Does the LH Surge Last?
The duration of the LH surge is one of the most variable aspects of the menstrual cycle, often lasting between 12 and 48 hours from its onset to the return to baseline. Ovulation generally occurs 24 to 36 hours after the initial onset of the LH surge is detected. More specifically, the mature egg is typically released about 10 to 12 hours after the LH concentration reaches its peak.
The timing can vary significantly, even from one cycle to the next for the same woman. Some women experience a very rapid, short surge, which makes it easy to miss if testing is not frequent. Other women may have a plateau pattern where LH levels stay high for two to three days before dropping. Variability in the surge’s duration means that a longer surge offers a wider window of time to successfully detect the hormonal change.
Practical Application: Tracking the Surge with Test Kits
Ovulation predictor kits (OPKs) work by detecting the presence of luteinizing hormone in the urine. These at-home tests turn positive when LH levels cross a certain threshold, indicating the start of the surge. Since the surge can be brief, testing at the right time of day is important to avoid missing the window.
Most kits recommend testing in the mid-afternoon (10 a.m. to 8 p.m.) because LH is synthesized in the morning and takes time to appear in the urine. Limiting fluid intake for a couple of hours before testing is advisable, as diluted urine can lead to a false negative result. To ensure the short surge is not missed, it is often recommended to test twice a day once the surge is anticipated. A positive result occurs when the test line is as dark as or darker than the control line. Following a positive result, couples are advised to time intercourse within the next 24 to 48 hours to align with the estimated ovulation time.